Flame structure and chemiluminescence in premixed flames

摘要

The quantitative use of chemiluminescence requires the knowledge of the relationship between the concentration of excited species with goal flame properties, such as heat release rate. To find them, we have analyzed, numerically and analytically, the distribution of the excited chemiluminescent species OH~* and CH~* in steady hydrogen and methane planar premixed flames. The results show that both OH~* and CH~* are in steady state to a very good degree of approximation, so their concentrations are given by the balance between production and consumption. The analyses show that OH~*, in both flames, is broadly distributed downstream the preheat region: OH~* is initially produced irreversibly, and reaches super-equilibrium concentrations in a thin non-equilibrium region located in the fuel consumption layer where the concentrations of O and H, as well as that of OH~*, are building up; further downstream, in the high-temperature region of the flame, OH~* relaxes to equilibrium with OH, with concentrations several orders of magnitude smaller than those in the non-equilibrium region. Conversely, CH~* is everywhere in equilibrium with CH and therefore can be found only in the fuel consumption layer in lean flames, but it is broadly distributed, as CH, in the case of rich flames.